That's true, but the (poorly expressed) analogy is specifically referring to the security guard's decision whether to leave his office and investigate a noise.

That's true, but the (poorly expressed) analogy is specifically referring to the security guard's decision whether to leave his office and investigate a noise. Of course, the mere existence of the security guard in an office (if known to potential burglars) does likely provide some deterrent, but I think the analogy is referring specifically to cases where a burglar would make an attempt regardless of the burglar's knowledge of the existence of a security guard.